Not Applicable
Not Applicable
1. Field of the Invention
The present invention relates to a method for forming a training sequence in communication in a communication system where information is transmitted at one or several carrier frequencies, the training sequence being used at least as a synchronization signal. The invention relates further to a transmitter for transmitting information in one or several data frames at one or several carrier frequencies, the transmitter comprising means for forming at least one training sequence and attaching the same to the data frame to be transmitted, and the training sequence being intended for synchronizing the receiver. Furthermore, the invention relates to a receiver comprising means for synchronizing the receiver to a received signal which at the transmitting stage is provided with at least one training sequence, as well as a communication system comprising a transmitter for transmitting information into a communication channel in one or several data frames, and means for forming at least one training sequence and attaching the same to a data frame to be transmitted, a receiver for receiving transmitted information from the communication channel, and means for synchronizing the receiver.
2. Description of the Related Art Including Information Disclosed Under 37 CRF 1.97 and 1.98
Transmission of signals from a transmitter to a receiver via the radio channel is subject to a number of interference factors. For example, weather changes cause so-called fading, wherein at the receiving end, the strength of the signal varies and may prevent correct reception of information. Other interference factors include channel noise and ignition interference caused by electric devices. Particularly in urban areas, one significant interference factor affecting the reliability of data transmission is multipath propagation which is caused by the fact that the signal enters the receiver via several different routes, and the path traveled by these different signals can differ in length. As a result, the different signals enter the receiver in different phases, thereby disturbing correct transmission of information. The situation is even more complicated when the transmitter or the receiver is moving, because the situation changes all the time, wherein synchronization of the receiver is difficult.
One method for reducing the effect of these said interference factors is to transmit information in parallel. Thus, the bits of information to be transmitted are divided into groups, and each group is demodulated at subcarriers of different wavelengths. At the receiving end, these signals of different carrier frequencies are received and modulated, and subsequently connected to a bit string corresponding to the original information that was transmitted. In such a method, it is possible to reduce the bit rate of each group and still obtain the same final bit rate. For example, in a situation where eight subcarriers are used, the bit rate of each group can be reduced to an eighth part. Thus, particularly interference of short duration affect the transmission of information substantially less than in a situation where information is transmitted in series mode, i.e. modulated by one carrier. One method used for such serial transmission of information is the so-called OFDM modulation technique, i.e. orthogonal frequency division multiplexing. Below in this description, this technique will be referred to by the abbreviation OFDM.
The information to be transmitted can be any information as such, including audio signals, video signals, data files, text messages, etc. However, information in analog form is converted into binary form with an analog/digital converter before modulation. The binary information is divided into blocks of preferably fixed size. Each of these blocks is coded and transmitted as one data frame. Usually, such a block is further divided into smaller groups, each group consisting of typically two to five bits. Each of these groups is modulated at one subcarrier frequency. In the modulation, the amplitude and phase of the subcarrier frequency is set to a value determined on the basis of the value of the bits in the group. Thus, in a situation where the group consists of m bits, the modulation results in 2m different alternatives for the phase and the amplitude. In this description, these alternatives are called the constellation. For example, in quadrature amplitude modulation (QAM), the size of the modulating group is given in the form 2m-QAM, for example 4-QAM, 16-QAM or 64-QAM. It is obvious that also other modulation techniques can be used in connection with this invention.
In the receiver, each subcarrier frequency is demodulated to find out which symbol was transmitted in each group. This is conducted by examining from the received signal at each subcarrier frequency the phase and amplitude of the signal, to decide which signal point is closest to the received signal point. This signal point determines which symbol was probably transmitted. Subsequently, the original block can be reconstructed by combining the groups received at different sub-carrier frequencies and demodulated.
Irrespective of whether the information was coded according to either or both the phase and the amplitude of the signal transmitted, it is necessary to synchronize the receiver with the received signal. Typically, each block must be synchronized independently, because in many systems the receiver does not know the exact moment of transmission. Therefore, each block to be transmitted must include synchronization data, on the basis of which the receiver tries to synchronize itself with the received signal and to find out the information transmitted. To eliminate phase and frequency errors, the transmitter modulating frequencies should be kept exactly constant, and correspondingly, the demodulation frequencies in the receiver should be at the correct frequency and the frequency should be as constant as possible, which requires the use of high-quality components as well as relatively complicated circuits. Known synchronization methods are often based on the use of training components which are typically identical. In synchronization, it is known to use an optimal matched filter (MF) whose coefficients are matched with the training parts of the received signal. The coefficients are usually fixed and they must be known to the receiver. The design of such matched filters requires complicated computing, and furthermore, each OFDM symbol must be equipped at the transmission stage with a guard time to eliminate the effect of multipath propagation. In the case of a matched filter, multipath propagation is not taken into account in the filter coefficients. If the guard time is not added to the OFDM symbol, the output of the matched filter includes extra peaks formed by incoming signals in the receiver at different delay times. Another known solution is the use of a simpler synchronizer, where subsequent similar training parts are correlated with each other, wherein multipath propagation of the channel affects both of the symbols to be correlated in the same way. The receiver does not need to know the training parts.
In a simpler receiver synchronizer, a correlation of the training parts is determined, resulting in a step function in the case that the training parts are identical. However, it is very difficult to deduce the correct timing and frequency, particularly with varying power levels of the signal to be received. Timing and frequency errors are caused by different unideal conditions, the oscillator, the communication channel, etc. Bit errors occur if timing and frequency errors are not corrected in the receiver. In the case of a timing error, Fourier conversion is made at a wrong location. The frequency error leads to rotation of signal points (constellation points). The transmitter and the receiver must be synchronized with each other, to eliminate bit errors.
Several synchronization methods are known, but most of them are based on correlation between the received signal and a known training sequence consisting of one or several identical/training parts, or on correlation between the guard time and the corresponding OFDM symbol. Furthermore, there are synchronization methods in which some of the OFDM subcarriers are used for synchronization only (so-called pilot carriers).
It is a purpose of the present invention to provide a method for forming training parts in such a way that the synchronization of the receiver with the transmitted signal is improved with respect to using training parts of prior art. The invention is based on the idea that the training sequence is formed in such a way that correlation of two subsequently received training sequences results in at least one peak, preferably an alternating positive and negative peak. This is achieved preferably in such a way that at least two subsequent training parts in the training sequence are identical, and at least one training part is the negation of these. The method according to the present invention is characterized in that the training sequence is composed of three or more training parts in such a way that in the training sequence:
at least two adjacent training parts are formed substantially identical, and furthermore, at least one training part is formed substantially as a negation to said identical training parts, or
at least two adjacent training parts are formed substantially identical, and furthermore, at least one training part is formed substantially different from said identical training parts, or
at least two adjacent training parts are formed substantially as negations to each other, and furthermore, at least one training part is formed substantially different.
The transmitter according to the present invention is characterized in that the transmitter comprises means for forming the training sequence from three or more training parts in such a way that
at least two adjacent training parts are formed substantially identical, and furthermore, at least one training part is formed substantially as a negation to said identical training parts, or
at least two adjacent training parts are formed substantially identical, and furthermore, at least one training part is formed substantially different from said identical training parts, or
at least two adjacent training parts are formed substantially as negations to each other, and furthermore, at least one training part is formed substantially different.
The receiver of the present invention is characterized in that the means for synchronizing the receiver comprise:
means for forming a correlation function on the basis of the received training sequence,
means for finding at least one maximum or minimum point in the correlation function, and
means for determining the timing moment and the phase error on the basis of at least one found maximum or minimum point.
Furthermore, the communication system according to the present invention is characterized in that the communication system comprises further means for forming a training sequence of three or more training parts in such a way that
at least two adjacent training parts are formed substantially identical, and furthermore, at least one training part is formed substantially as a negation to said identical training parts, or
at least two adjacent training parts are formed substantially identical, and furthermore, at least one training part is formed substantially different from said identical training parts, or
at least two adjacent training parts are formed substantially as negations to each other, and furthermore, at least one training part is formed substantially different.
The present invention gives significant advantages to solutions of prior art. From the received signal, it is easy to determine the correct timing and the carrier frequency error, for example for the reason that a positive maximum and a negative maximum are preferably alternating in the correlation function. In the communication system according to the invention, the receiver can be synchronized better and timing and frequency errors can be corrected more accurately than in communication systems of prior art. Moreover, the method according to the invention is also less sensitive to multipath propagation than solutions of prior art.